1. Field of the Invention
The present invention relates to a dielectric filter comprising a plurality of juxtaposed dielectric resonators arranged in a dielectric ceramic block.
2. Prior Art
There are known dielectric filters of the type comprising a rectangularly parallelepipedic dielectric ceramic block, three or more than three resonators formed by boring so many through holes in the dielectric ceramic block and coating the peripheral walls thereof with an inner conductor and an outer conductor covering the outer peripheral surface of the dielectric ceramic block, except one end surface of the dielectric ceramic block which is intended to function as an open circuit end on which one opening ends of the through holes are arranged.
FIGS. 1 and 2 of the accompanying drawings show a conventional dielectric filter of the above identified type comprising a dielectric ceramic block A provided with three resonators B and C and an outer conductor D, wherein a pair of input/output pads P are formed on the peripheral surface portions of the dielectric ceramic block A, arranged opposite to each other at locations close to the short-circuiting end surface of the dielectric ceramic block A and facing the respective outer resonators C and electrically connected to the respective outer resonators C by way of respective conductive holes E each of said input/output pads P being separated from the outer conductor D by an insulating zone F surrounding it.
While the resonators B and C are normally made to have a length equal to .lambda./4 or a quarter of the specified resonant frequency, the above arrangement of input/output pads P disposed vis-a-vis the respective outer resonators C at locations close to the short-circuiting end surface gives rise to a problem that the resonant frequency of the outer resonators C is decreased due to the provision of the input/output pads P separated from the outer conductor D by respective insulating zones F and the partial removal of the outer conductor D and consequently the resonant frequency of the outer resonators C comes to disagree with that of the central resonator B at the cost of filtering performance.
This problem may be dissolved by providing a projection G in a central area of the bottom side surface of the dielectric ceramic block A as shown in FIGS. 1 and 2, which is the short-circuiting end of the dielectric ceramic block A so as to make the resonant length of the outer resonators C shorter than that of the central resonator B and shift the resonant frequency of the resonators C upward in advance in order to compensate the lowered resonant frequency of the outer resonators C caused by the input/output pads P and make the resonant frequencies of all the resonators B and c consequently agree with each other.
It should be appreciated, however, that a conductive film H is formed on the short-circuiting end surface of the conventional dielectric filter and connected to the edges of the openings of the resonators on that side. The conductive film H is typically prepared by screen printing which is adapted to mass production. However, with the configuration of the dielectric filter of FIGS. 1 and 2 having a projection G formed in a central area of the short-circuiting end surface, the screen printing technique cannot feasibly be used and the conductive film H has to be formed by applying a conductive material to that side by means of a brush at the cost of manufacturing efficiency. In short, such a configuration is not adapted to mass production.
It is, therefore, an object of the present invention to provide a dielectric filter that can make the resonant frequencies of the resonators agree with each other and, at the same time, electrically connect the input/output pads and the respective outermost resonators without requiring the formation of a projection on the short-circuiting end surface of a dielectric ceramic block.